Abstract: The recent interest in using microorganisms for biofuels is motivation enoughto study bioconvection and cell dispersion in tubes subject to imposed flow. Tooptimize light and nutrient uptake, many microorganisms swim in directionsbiased by environmental cues e.g. phototaxis in algae and chemotaxis inbacteria. Such taxes inevitably lead to accumulations of cells, which, as manymicroorganisms have a density different to the fluid, can induce hydrodynamicinstabilites. The large-scale fluid flow and spectacular patterns that ariseare termed bioconvection. However, the extent to which bioconvection isaffected or suppressed by an imposed fluid flow, and how bioconvectioninfluences the mean flow profile and cell transport are open questions. Thisexperimental study is the first to address these issues by quantifying thepatterns due to suspensions of the gravitactic and gyrotactic greenbiflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow.With no flow, the dependence of the dominant pattern wavelength at patternonset on cell concentration is established for three different tube diameters.For small imposed flows, the vertical plumes of cells are observed merely tobow in the direction of flow. For sufficiently high flow rates, the plumesprogressively fragment into piecewise linear diagonal plumes, unexpectedlyinclined at constant angles and translating at fixed speeds. The patternwavelength generally grows with flow rate, with transitions at critical ratesthat depend on concentration. Even at high imposed flow rates, bioconvection isnot wholly suppressed and perturbs the flow field.